This invention relates to an apparatus and method for processing sewage scum; in particular, the invention relates to an apparatus and method for processing the collected primary skimmings, for example, from wastewater treatment plants. The purpose of this process is to separate and treat the solids portion of these skimmings so that they are acceptable for ordinary landfill disposal, to separate and treat the fats and oils portion of the skimmings for sale as a raw product for distillation, or for incineration and recovery of heat values and to return the oil-and solid-free water back into the treatment plant influent. Alternatively, primary skimmings are treated to remove solids followed by microbial or other digestion of the remainder and return of the digest to the treatment plant influent.
One of the first steps in the treatment of municipal sewage or raw sewage from other sources is skimming an upper layer from the raw sewage for disposal prior to treating the sewage for discharge, by flocculation, aerobic digestion, or other techniques. Typically, a layer of scum rises to the surface of the raw sewage when it is, for example, held in a settling tank or basin. This scum is typically skimmed from the surface of the raw sewage for separate treatment to prevent interference by the materials in the scum with treatment processes applied to the remainder of the sewage. Removal of this scum layer is typically accomplished with a primary skimmer. The scum thus removed from the raw sewage typically contains about 70% water, about 15% plastics and other solids, and about 15% fats, oils, and other oily substances. Until recently, the scum removed from the raw sewage in this way was either disposed of by dumping or landfilling the scum, possibly after the removal of the aqueous component of the scum, or by drying and incinerating or burning the scum.
With increased interest in the environment and increased regulatory requirements for pollution abatement equipment, such methods of scum disposal are no longer practicable. Dumping or landfilling of scum is undesirable due to the potentially harmful environmental consequences, the continually increasing high cost of landfilling, and the increasingly limited locations in which such disposal is permitted. Burning of dewatered scum is possible, but, because of the very high incinerator temperatures that are required to completely combust the plastics typically found in sewage scum, burning of large quantities of scum in a way that will meet regulatory requirements is generally infeasible.